Battery pack of electric power tool

ABSTRACT

A battery pack of an electric power tool comprises ten lithium-ion cells. The ten lithium-ion cells are connected in series. Each lithium-ion cell has a diameter equal to or less than 18 millimeters, a length equal to or less than 65 millimeters, and an internal resistance equal to or less than 30 milliohms. Because the battery pack has a high voltage in operation and is therefore able to supply large current, the battery pack is preferably configured incapable of being used in a conventional electric power tool that cannot operate under such a large current.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Japanese Patent Application No.2010-192239 filed on Aug. 30, 2010 and U.S. Provisional Application No.61/379,984 filed on Sep. 3, 2010, the contents of which are herebyincorporated by reference into the present application.

TECHNICAL FIELD

The present invention relates to a battery pack of an electric powertool, and more particularly to a battery pack having lithium-ion cells.

DESCRIPTION OF RELATED ART

JP 2008-518798 A discloses an electric power tool powered by a batterypack. The battery pack of this electric power tool has a plurality oflithium-ion cells. The lithium-ion cell makes it possible to obtain ahigh output voltage and merits a high energy density. With the techniquedescribed in JP 2008-518798 A, since the lithium-ion cells are used inthe battery pack, an output-to-weight ratio of the electric power tool(including the battery pack; same hereinbelow) is increased.

SUMMARY OF THE INVENTION

It is preferable that an electric tool has a high output-to-weight ratioor output-to-volume ratio. For this reason, it is necessary not only touse lithium-ion cells in a battery pack, but also to increase theoutput-to-weight or output-to-volume ratio of the electric power tool.

To meet the above-mentioned demand, it is necessary to increase theoutput voltage of the battery pack, without changing the size or numberof the lithium-ion cells used therein. Thus, in accordance with thepresent invention, lithium-ion cells with a small internal resistanceare used in the battery pack of the electric power tool. Where theinternal resistance of a lithium-ion cell is small, the internal voltagedrop in the lithium-ion cell decreases. Since a comparatively highcurrent flows in electric power tools, the internal voltage drop causedby the lithium-ion cells also becomes comparatively large. Inparticular, in a battery pack in which a plurality of lithium-ion cellsis connected in series, the internal voltage drop caused by thelithium-ion cells can surpass the output voltage of one lithium-ion cellor the plurality of lithium-ion cells. By suppressing such an internalvoltage drop, it is possible to increase the working voltage of thebattery pack (output voltage during conduction), without changing thesize or number of the lithium-ion cells used therein. Thus, theoutput-to-weight or output-to-volume ratio of the electric power toolcan be increased.

The inventors have verified that lithium-ion cells having an internalresistance equal to or higher than 30.3 milliohm have been used in thebattery packs of conventional electric power tools. Therefore, by usinglithium-ion cells having an internal resistance equal to or less than 30milliohm, it is possible to increase the output-to-weight andoutput-to-volume ratio over those of the conventional products. Theabovementioned value of 30.3 milliohm is obtained by measuring theinternal resistance of the conventional lithium-ion cell having adiameter of 18 millimeters and a length of 65 millimeters. Therefore, inaccordance with the present invention, a lithium-ion cell that has adiameter equal to or less than 18 millimeters and a length equal to orless than 65 millimeters and also has an internal resistance equal to orless than 30 milliohm is preferred.

On the basis of the above-described findings, it is preferred that thebattery pack for an electric power tool disclosed in the presentdescription have at least one lithium-ion cell, wherein the lithium-ioncell has an internal resistance equal to or less than 30 milliohms. Inthis case, it is preferred that the lithium-ion cell have a diameterequal to or less than 18 millimeters and a length equal to or less than65 millimeters. With such a configuration, the output of the batterypack can be increased, without changing the size or number oflithium-ion cells with respect to that of the conventional product. As aresult, the output-to-weight and output-to-volume ratio of the electricpower tool can be increased.

In the above-described electric power tool, lithium-ion cells of a smallsize can be used. Usually, the decrease in size of lithium-ion cellsresults in the increased internal resistance thereof. Where the diameterof the above-mentioned conventional lithium-ion cell (diameter 18millimeters, length 65 millimeters, internal resistance 30.3 milliohms)is reduced to 14 millimeters, the internal resistance thereof becomes49.2 milliohms. Alternatively, where the length of the conventionallithium-ion cell is assumed to decrease to 45 millimeters, the internalresistance thereof becomes 43.6 milliohms. Therefore, where alithium-ion cell with a diameter equal to or less than 14 millimetersand a length equal to or less than 65 millimeter is used, it ispreferred that this lithium-ion cell have an internal resistance equalto or less than 49 milliohms. Further, where a lithium-ion cell with adiameter equal to or less than 18 millimeters and a length equal to orless than 45 millimeter is used, it is preferred that this lithium-ioncell have an internal resistance equal to or less than 40 milliohms. Asa result, the output-to-weight and output-to-volume ratio of theelectric power tool can be increased with respect to those of theconventional product.

It is preferable that each of the abovementioned battery packs has aplurality of lithium-ion cells connected in series. Where the pluralityof lithium-ion cells is connected in series, the working voltage of thebattery pack can be increased. Even when the plurality of lithium-ioncells is connected in series, since the internal resistance of eachlithium-ion cell is comparatively small, so the internal resistance ofthe battery pack can be suppressed to a comparatively low value.

For example, a battery pack having ten lithium-ion cells connected inseries can be used. In this case, the nominal voltage of the batterypack can be 36 V and the nominal capacity of the battery pack can beequal to or higher than 1 Ampere-hour.

As described hereinabove, where lithium-ion cells with a low internalresistance are used, the working voltage of the battery pack rises andthe output of the electric power tool can be increased. However, wherethe battery pack using the lithium-ion cells with a small internalresistance is used as a power supply of the conventional electric powertool, a high current supplied by the battery pack can damage theconstituent components of the conventional electric power tool. For thisreason, the battery pack in accordance with the present invention shouldbe used only with specially designed electric power tools, and it ispreferred that the use for the conventional electric power tool beprohibited. Meanwhile, the electric power tool that can use the batterypack in accordance with the present invention can also use, without anyproblem, the battery packs of the conventional electric power tools.

Based on the above-described findings, the present invention providesthe below-described electric power tool system. This electric power toolsystem includes a first electric power tool powered by a first batterypack and a second electric power tool powered by a second battery pack.The first and second battery packs include the same number oflithium-ion cells. The first battery pack accommodates lithium-ion cellswith an internal resistance lower than that of the lithium-ion cells ofthe second battery pack. Thus, the first battery pack corresponds to theabove-described battery pack in accordance with the present invention,and the second battery pack corresponds to the battery pack of theconventional electric power tool. In this case, it is preferred that thefirst battery pack be configured capable of being detachably attached tothe main body of the first electric power tool. It is also preferredthat the second battery pack be configured capable of being detachablyattached to the main body of the second electric power tool and alsocapable of being detachably attached to the main body of the firstelectric power tool.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of an electric power tool according to anembodiment.

FIG. 2 shows a battery receiving portion of the electric power toolaccording to the embodiment, as viewed from the front surface (from theleft side in FIG. 1).

FIG. 3 is a perspective view of a battery pack of the electric powertool according to the embodiment.

FIG. 4 is a front view of the battery pack of the electric power toolaccording to the embodiment.

FIG. 5 illustrates schematically the compatibility of the battery packwith the electric power tool according to the embodiment and theconventional electric power tool.

FIG. 6 is a graph illustrating the results obtained in measuring theinternal resistance of a lithium-ion cell.

FIG. 7 illustrates measurement conditions for the internal resistance ofthe lithium-ion cell.

FIG. 8 illustrates outer dimensions and effective dimensions of thelithium-ion cell. FIG. 8(A) shows schematically a vertical section ofthe lithium-ion cell. FIG. 8(B) is a cross-sectional view taken alongthe B-B line in FIG. 8(A) that shows schematically the transversesection of the lithium-ion cell.

FIG. 9 shows a battery receiving portion of the conventional electricpower tool, as viewed from the front surface.

FIG. 10 is a perspective view of a battery pack of the conventionalelectric power tool.

DETAILED DESCRIPTION OF THE INVENTION

Representative, non-limiting examples of the present invention will nowbe described in further detail with reference to the attached drawings.This detailed description is merely intended to teach a person of skillin the art further details for practicing preferred aspects of thepresent teachings and is not intended to limit the scope of theinvention. Furthermore, each of the additional features and teachingsdisclosed below may be utilized separately or in conjunction with otherfeatures and teachings to provide improved battery packs for electricpower tools, as well as methods for using and manufacturing the same.

Moreover, combinations of features and steps disclosed in the followingdetail description may not be necessary to practice the invention in thebroadest sense, and are instead taught merely to particularly describerepresentative examples of the invention. Furthermore, various featuresof the above-described and below-described representative examples, aswell as the various independent and dependent claims, may be combined inways that are not specifically and explicitly enumerated in order toprovide additional useful embodiments of the present teachings.

All features disclosed in the description and/or the claims are intendedto be disclosed separately and independently from each other for thepurpose of original written disclosure, as well as for the purpose ofrestricting the claimed subject matter, independent of the compositionsof the features in the embodiments and/or the claims. In addition, allvalue ranges or indications of groups of entities are intended todisclose every possible intermediate value or intermediate entity forthe purpose of original written disclosure, as well as for the purposeof restricting the claimed subject matter.

An electric power tool 10 according to an embodiment will be explainedbelow with reference to the drawings. As shown in FIGS. 1 and 2, theelectric power tool 10 is provided with a main body 12 and a batterypack 50 that supplies power to the main body 12 of the electric powertool 10. A tool holder 14 that can be detachably attached to the tool, amain switch 16 operated by the user, and a grip 18 held by the user areprovided at the main body 12. A battery receiving portion 20 that candetachably receive the battery pack 50 is provided at the lower end ofthe grip 18. A motor or a circuit substrate (not shown in the figure)for driving the tool holder 14 is accommodated inside the main body 12.The electric power tool 10 of the present embodiment is, by way ofexample, an electric power driver, and a driver bit (not shown in thefigure) is mounted on the tool holder 14.

The battery pack 50 has a housing 52 and ten lithium-ion cells 70 housedin the housing 52. The ten lithium-ion cells 70 are electricallyconnected in series. The nominal voltage of each lithium-ion cell 70 is3.6 V. Therefore, the nominal voltage of the entire battery pack 50 is36 V. The rated voltage of the electric power tool 10 is also 36 V. Aconnector portion 60 is provided at the upper surface of the housing 52.The connector portion 60 is detachably engaged with the batteryreceiving portion 20 of the main body 12.

As shown in FIG. 2, a pair of rails 22 is formed at the batteryreceiving portion 20 of the main body 12. Further, as shown in FIGS. 3and 4, a pair of rails 62 is also formed in the connector portion 60 ofthe battery pack 50. The pair of rails 62 of the battery pack 50 isslidably engaged with the pair of rails 22 of the main body 12. As aresult, the battery pack 50 is physically connected to the main body 12.

As shown in FIG. 2, the battery receiving portion 20 of the main body 12is provided with a positive terminal 26 and a negative terminal 28. Thepositive terminal 26 and the negative terminal 28 are electricallyconnected to the circuit board and motor located inside the main body12. As shown in FIGS. 3 and 4, the connector portion 60 of the batterypack 50 is provided with a positive terminal 66 and a negative terminal68. The positive terminal 66 and the negative terminal 68 areelectrically connected to the ten lithium-ion cells 70 that areconnected in series. Where the battery pack 50 is attached to thebattery receiving portion 20 of the main body 12, the positive terminal66 and the negative terminal 68 of the battery pack 50 are electricallyconnected to the positive terminal 26 and the negative terminal 28,respectively, of the main body 12. As a result, the battery pack 50 iselectrically connected to the main body 12, and the discharge power ofthe ten lithium-ion cells 70 is supplied to the circuit board and motorlocated inside the main body 12.

The attachment structure of the battery pack in the electric power tool10 according to the above-described embodiment is essentially similar tothe attachment structure of a battery pack 150 in a conventionalelectric power tool 110 shown in FIGS. 9 and 10. Thus, in theconventional electric power tool 110, a battery receiving portion 120 ofa main body 112 is also provided with a pair of rails 122, a positiveterminal 126, and a negative terminal 128, and a housing 152 of thebattery pack 150 is provided with a pair of rails 162, a positiveterminal 166, and a negative terminal 168. In this case, similarly tothe battery pack 50 of the present embodiment, the conventional batterypack 150 shown in FIG. 10 has ten lithium-ion cells connected in series,and the nominal voltage thereof is 36 V. Thus, the rated voltage of theconventional electric power tool 110 is also 36 V.

As shown in FIGS. 2, 3, and 4, in the electric power tool 10 of thepresent embodiment, a rib 64 is formed at the connector portion 60 ofthe battery pack 50, and a groove 24 for receiving the rib 64 is formedat the battery receiving portion 20 of the main body 12. By contrast, asshown in FIGS. 9 and 10, in the conventional electric power tool 110, noportion corresponding to the abovementioned rib 64 is present at theconnector portion 160 of the battery pack 150, and no site correspondingto the abovementioned groove 24 is present at the battery receivingportion 120 of the main body 112. Therefore, as shown schematically inFIG. 5, the battery pack 50 of the present embodiment is configuredattachable only to the main body 12 of the present embodiment and cannotbe attached to the main body 112 of the conventional electric power tool110. By contrast, the main body 12 of the present embodiment isconfigured suitable for use not only with the battery pack 50 of thepresent embodiment, but also with the battery pack 150 of theconventional electric power tool 110.

As compared with the conventional electric power tool 110, the electricpower tool 10 of the present embodiment uses new lithium-ion cells 70with a low internal resistance. The internal resistance of the newlithium-ion cell 70 is improved to 26.8 milliohms. The new lithium-ioncell 70 is a cylindrical lithium-ion cell and has a diameter of 18millimeters and a length of 65 millimeters. By using the lithium-ioncells 70 with a low internal resistance, it is possible to inhibit thedecrease in internal voltage of the battery pack 50 during conduction.As a result, the working voltage of the battery pack 50 is increased andthe output performance of the electric power tool 10 is improved. Morespecifically, since a high current is supplied from the battery pack 50to the main body 12, the maximum torque that can be outputted by theelectric power tool 10 is increased. Furthermore, since the power lossin the battery pack 50 is reduced, the interval in which the electricpower tool 10 can be used is extended.

The nominal voltage of the battery pack 50 of the present embodiment isequal to the nominal voltage of the conventional battery pack 150, butthe actual output voltage during conduction of the battery pack 50 ofthe present embodiment is higher than that of the conventional batterypack 150. Therefore, where the battery pack 50 of the present embodimentis used in the conventional electric power tool 110, the electriccurrent supplied by the battery pack 50 can exceed the level allowed forthe main body 112 of the conventional electric power tool 110 andconstituent components thereof can be damaged. For this reason, asmentioned hereinabove, the battery pack 50 of the present embodiment isconfigured such that the attachment thereof to the main body 112 of theconventional electric power tool 110 is impossible and the use thereofon the conventional electric power tool 110 is prohibited (see FIG. 5).

The inventors have measured the internal resistance for three productsA, B, C of the conventional lithium-ion cells that have been developedfor electric power tools. The conventional three products A, B, C areall cylindrical lithium-ion cells and have a diameter of 18 millimetersand a length of 65 millimeters. According to the measurement resultsobtained by the inventors, the internal resistance of the conventionalproduct A is 30.3 milliohms, the internal resistance of the conventionalproduct B is 40.0 milliohms, and the internal resistance of theconventional product C is 50.9 milliohms. These measurement resultsconfirmed that among the conventional cylindrical lithium-ion cells thathave been used in electric power tools, there is no cell having adiameter equal to or less than 18 millimeters, a length equal to or lessthan 65 millimeters, and an internal resistance equal to or less than 30milliohms. The internal resistance of lithium-ion cells slightly variesdepending on the measurement method used. A method for measuring theinternal resistance according to the present description will beexplained below in greater detail.

In the electric power tool 10 of the abovementioned embodiment, the size(diameter: 18 millimeters and length: 65 millimeters) of the newlithium-ion cells 70 can be reduced. Typically, where the size oflithium-ion cells is reduced, the internal resistance thereof increases.FIG. 6 shows the relationship between the diameter of a lithium-ion cell(abscissa) and the internal resistance thereof (ordinate). In FIG. 6, agraph X shows the internal resistance of the new lithium-ion cell 70, agraph A shows the internal resistance of the conventional product A, agraph B shows the internal resistance of the conventional product B, anda graph C shows the internal resistance of the conventional product C.Among the internal resistance values shown in FIG. 6, the internalresistance value at a diameter of 18 millimeters is a measured value,and the values at other diameters are estimated values determined bycalculations from the measured value. The method for calculating theestimated values will be described below in greater detail.

As shown in FIG. 6, the internal resistance increases as the diameter ofthe lithium-ion cell decreases. However, in the case of the newlithium-ion cell 70 used in the present embodiment, although the celldiameter is reduced to 14 millimeters, the internal resistance thereofis restricted to a value equal to or less than 43.1 milliohms. Thisvalue is on par with the internal resistance of the conventionalproducts A, B, C having the diameter of 18 millimeters. Therefore, withthe new lithium-ion cells 70 used in the present embodiment, it ispossible to reduce the size and weight of the electric power tool 10including the battery pack 50, while maintaining the output performanceidentical to that of the conventional electric power tool 110. In thecase of the conventional lithium-ion cells A, B, C, where the diameteris reduced to 14 millimeters, the internal resistance increases tobecome equal to or greater than 49.2 milliohms.

When the size and weight of the electric power tool 10 are to bereduced, not only the diameter of the lithium-ion cells 70, but also thelength of the lithium-ion cells 70 may be decreased. In the case of thelithium-ion cells 70 used in the present embodiment, even when thelength is reduced to 45 millimeters, the internal resistance isrestricted to a value equal to or less than 38.3 milliohms. This valueis on par with the internal resistance of the conventional products A,B, C with the length of 65 millimeters. Therefore, with the newlithium-ion cells 70 used in the present embodiment, the size and weightof the electric power tool 10 including the battery pack 50 can bereduced, while maintaining the output performance identical to that ofthe conventional electric power tool 110. In the case of theconventional lithium-ion cells, A, B, C, where the length is reduced to45 millimeters, the internal resistance thereof is increased to at least43.6 milliohms. These values are also estimated values calculated by thebelow-described calculation method.

As described hereinabove, with the electric power tool 10 of the presentembodiment, where the new lithium-ion cells 70 with a low internalresistance are used, the output performance of the electric power tool10 is improved without increasing the size or weight of the electricpower tool 10. Alternatively, by decreasing the new lithium-ion cells 70in size, it is possible to decrease the size or weight of the electricpower tool 10, while maintaining the output performance identical tothat of the conventional electric power tool 110. Further, since thepower loss in the battery pack 50 is reduced, the interval in which theelectric power tool 10 can be used is extended. Further, even when thebattery pack 50 is charged, the power loss is small and the increase intemperature of the new lithium-ion cells 70 is suppressed.

(Method for Measuring the Internal Resistance)

A method for measuring the internal resistance in the present embodimentwill be explained below. With this measurement method, first, adischarge depth (charge level) of a lithium-ion cell is set to 50%. Morespecifically, after the lithium-ion cell is appropriately charged, thequantity of electricity corresponding to a half of the nominalcapacitance is discharged. The terminal voltage of the lithium-ion cellis then measured, while discharging the lithium-ion cell. In this case,the ambient temperature is maintained at 25±−1° C., and the temperatureof the lithium-ion cell is also 25±1° C. As shown in FIG. 7, when thelithium-ion cell is discharged, the discharge current changes with time.More specifically, in the first 1 sec, the discharge current is adjustedto 10 A, and in the subsequent 5 sec, the discharge current is adjustedto 20 A. The specific feature of this measurement method is that thelithium-ion cell is discharged at a high current, with consideration forthe use in an electric power tool. While the lithium-ion cell is thusdischarged, the current C1 and voltage V1 are measured after 1 sec haselapsed, and the current C2 and voltage V2 are measured after 6 sec haveelapsed. The internal resistance of the lithium-ion cell is determinedfrom the slope of the current-voltage line between the two points inwhich measurements have been conducted. Thus, the internal resistance Ris represented by the following formula.

R=(V1−V2)/(C2−C1)×1000 [mΩ]

(Method for Calculating the Estimated Value of Internal Resistance)

A method for calculating the estimated value of internal resistance inthe present embodiment will be explained below. As shown in FIG. 8, anelectrode assembly 72 having a positive electrode sheet and a negativeelectrode sheet wound with a separator being interposed therebetween isaccommodated in the lithium-ion cell 70, and the internal resistance ofthe lithium-ion cell 70 is inversely proportional to the surface area ofthe electrode assembly 72 spread on a plane. In this case, the surfacearea of the electrode assembly 72 spread on a plane is proportional tothe height h1 of the electrode assembly 72 accommodated in thelithium-ion cell 70 and also proportional to the transversecross-sectional area S of the electrode assembly 72 accommodated in thelithium-ion cell 70. In this case, the difference h2 between the heightH of the lithium-ion cell 70 and the height h1 of the electrode assembly72 is 7 millimeters. The diameter d1 of the cavity formed in the centerof the electrode assembly 72 is 5 millimeters. The share of structuralresistance fraction created by the collector or the like in the internalresistance of the lithium-ion cell 70 is 5 milliohms.

Therefore, it can be calculated that when the new lithium-ion cell 70having the diameter of 18 millimeters, the length of 65 millimeters, andthe internal resistance of 26.8 milliohms, is reduced in diameter to 14millimeters, the internal resistance thereof will become 43.1 milliohms.Alternatively, it can be calculated that when the new lithium-ion cell70 is reduced in length to 45 millimeters, the internal resistancethereof will become 38.3 milliohms. Likewise, it can be calculated thatwhen the lithium-ion cell that is the conventional product A having thediameter of 18 millimeters, the length of 65 millimeters, and theinternal resistance of 30.3 milliohms, is reduced in diameter to 14millimeters, the internal resistance thereof will become 49.2 milliohms.Alternatively, it can be calculated that when the lithium-ion cell thatis the conventional product A is reduced in length to 45 millimeters,the internal resistance thereof will become 43.6 milliohms.

The embodiments of the present invention are described above in detail,but these embodiments are merely exemplary and place no limitation onthe patent claims. Thus, the technical scope of the claims includesvarious modifications and changes of the above-described specificexamples. In particular, the actual diameter and length of the batterycells do not necessarily strictly match the dimensions (diameter: 18millimeters and length: 65 millimeters) explained in the presentdetailed description of the invention, and a certain difference indimensions (several millimeters) may be present.

What is claimed is:
 1. A battery pack for an electric power tool, thebattery pack comprising: at least one lithium-ion cell, wherein thelithium-ion cell has an internal resistance equal to or less than 30milliohms.
 2. The battery pack as in claim 1, wherein the lithium-ioncell has a diameter equal to or less than 18 millimeters and a lengthequal to or less than 65 millimeters.
 3. The battery pack as in claim 1,wherein the at least one lithium-ion cell includes a plurality oflithium-ion cells connected in series.
 4. The battery pack as in claim1, wherein the at least one lithium-ion cell includes ten lithium-ioncells connected in series, a nominal voltage of the battery pack isequal to 36 volts and a nominal capacity of the battery pack is equal toor more than 1 Ampere-hour.
 5. A battery pack for an electric powertool, the battery pack comprising: at least one lithium-ion cell,wherein the lithium-ion cell has a diameter equal to or less than 14millimeters, a length equal to or less than 65 millimeters, and aninternal resistance equal to or less than 49 milliohms.
 6. The batterypack as in claim 5, wherein the at least one lithium-ion cell includes aplurality of lithium-ion cells connected in series.
 7. The battery packas in claim 5, wherein the at least one lithium-ion cell includes tenlithium-ion cells connected in series, a nominal voltage of the batterypack is equal to 36 volts and a nominal capacity of the battery pack isequal to or more than 1 Ampere-hour.
 8. A battery pack for an electricpower tool, the battery pack comprising: at least one lithium-ion cell,wherein the lithium-ion cell has a diameter equal to or less than 18millimeters, a length equal to or less than 45 millimeters, and aninternal resistance equal to or less than 40 milliohms.
 9. The batterypack as in claim 8, wherein the at least one lithium-ion cell includes aplurality of lithium-ion cells connected in series.
 10. The battery packas in claim 8, wherein the at least one lithium-ion cell includes tenlithium-ion cells connected in series, a nominal voltage of the batterypack is equal to 36 volts and a nominal capacity of the battery pack isequal to or more than 1 Ampere-hour.
 11. An electric power tool systemcomprising: a first electric power tool powered by a first battery pack;and a second electric power tool powered by a second battery pack;wherein the first battery pack and the second battery pack comprise thesame number of lithium-ion cells, an internal resistance of thelithium-ion cell of the first battery pack is lower than an internalresistance of the lithium-ion cell of the second battery pack, the firstbattery pack is configured capable of being detachably attached to amain body of the first electric power tool but is configured incapableof being attached to a main body of the second electric power tool, andthe second battery pack is configured capable of being detachablyattached to the main body of the second electric power tool and isconfigured capable of being attached to the main body of the firstelectric power tool.
 12. The system as in claim 11, wherein thelithium-ion cell of the first battery pack has an internal resistanceequal to or less than 30 milliohms.
 13. The system as in claim 12,wherein the lithium-ion cell of the first battery pack has a diameterequal to or less than 18 millimeters and a length equal to or less than65 millimeters.
 14. The system as in claim 11, wherein the lithium-ioncell of the first battery pack has a diameter equal to or less than 14millimeters, a length equal to or less than 65 millimeters, and aninternal resistance equal to or less than 49 milliohms.
 15. The systemas in claim 11, wherein the lithium-ion cell of the first battery packhas a diameter equal to or less than 18 millimeters, a length equal toor less than 45 millimeters, and an internal resistance equal to or lessthan 40 milliohms.
 16. The system as in claim 11, wherein the at leastone lithium-ion cell of the first battery pack includes a plurality oflithium-ion cells connected in series.
 17. The system as in claim 11,wherein the at least one lithium-ion cell of the first battery packincludes ten lithium-ion cells connected in series, a nominal voltage ofthe first battery pack is equal to 36 volts and a nominal capacity ofthe first battery pack is equal to or more than 1 Ampere-hour.